The present invention relates to a method and an apparatus for cleaning filters contaminated by polymers and hot-melting resins in situ, without removing the filtering elements.
It is known that facilities for producing polymers and hot-melting resins, as well as facilities for processing and converting said products in the molten state, are often provided with filtration devices that have the purpose of removing from the product any impurities, gels, macromolecules, or various molecular aggregates that reduce the purity and quality of the polymer and/or of the resin.
The filtering elements of the filtration devices through which the flow of polymer or resin passes are usually contained in metal bodies that are adapted to be heated and to withstand significant pressures; it should be noted that the filtering elements and the corresponding containment bodies are usually rather expensive parts and it is therefore economically convenient to clean them in order to be able to reuse them.
The cleaning operations to which the filtering elements of currently commercially available filtration devices are subjected can consist in replacing the filtering element by means of an operation entailing partial disassembly of the containment body inside which the filtering element is placed.
This cleaning method can be performed almost exclusively on filters having a small surface, or on filters for coarse filtrations, and in facilities that process polymers and/or products in general that are not damaged if they are exposed to air.
Another cleaning method consists in fully replacing the filtering element and the containment body.
This method entails operations that are difficult and dangerous to perform, since it is necessary to work with bodies that are at a high temperature, generally around 200-300.degree. C. and are dirty and clogged with product, which tends to escape and drip from the filtering element.
These difficulties are particularly significant with filters having large surfaces, since they are bulky and very heavy, weighing even a few tons. Moreover, from a purely technological point of view, this operation has the drawback of causing the circulation of the polymer, contained in the passages of the mechanical parts that connect the filter to the facility, which has suffered a degradation that increases in proportion to the time that elapses between the interruption and the resumption of the flow of product inside the filter.
This operation also usually requires the availability of clean filters ready to perform replacement, with a significant increase in investment costs.
After removal, the filters must be subjected to a cleaning process in expensive, specifically provided facilities, where the residues of polymers and/or resins that contaminate the containment body and the filtering elements are removed.
Another method consists in cleaning the filtering element and the containment body in situ. This method requires the possibility of interrupting the flow of the product through the filter being cleaned for all the time required to complete the cleaning cycle. Therefore, cleaning in situ very often entails using filters having a double filtering body, which are connected to the product inlet and outlet ducts through valves allowing to divert the flow of product to be filtered into one filter while the other filter is cleaned or vice versa.
Processes for cleaning in situ, i.e., without removing the filtering elements or the filtering body, are currently rarely used in the field of polymers and hot-melting resins, since they can be performed only by circulating through the filtering bodies large amounts of specific solvents that are capable of dissolving the specific type of polymer or the type of resin being processed. This method has the drawback that it has high costs for purchasing the solvent and then for treating it or disposing of it. The presence of solvent in the processing department also causes environmental pollution problems.
The use of solvents, besides not achieving a fully satisfactory degree of cleaning because of the aggressive action of the solvent on the gels and on the macromolecules is weak, also entails the risk of solvents catching fire, thus require special safety measures in the execution of the facility.
These problems have so far limited the diffusion of the cleaning-in-situ method, despite the undisputed advantages that it entails, such as for example the elimination of mechanical interventions for disassembly and cleaning on hot and dirty filters, the elimination of separate facilities for cleaning the dirty filters, and the removal not only of the polymer contained in the filter but also of the polymer contained in the passages of the mechanical parts that connect the valves and the filter body.